1. Every time you move coins around it takes 101 blocks (roughly 101 minutes) before they mature and start staking again. Also due to randomness maybe you just had worse luck for a little while, try a bit longer.
If after a while you still believe it is slower after splitting it, we'd be curious to hear about it. You can always combine it back.

2. You can go to Privacy - Coin control and then click select all in that dialog. Or to Settings - Options - Wallet - Enable coin control features. Then the coin control button appears in the Send tab also.


See 2. above to see how many UTXOs you have.
Do note that first you need a big enough balance (see guidelines in the post) to bother splitting your balance into 100 stacks. This type of optimization is near useless without a big enough balance to split.